The process properties for DC plasma spraying are affected by the arc as the source of energy for plasma generation. For instance, the position of the anode attachment point, the arc movement and the arc formation are significantly influencing process stability, reproducibility and coating quality. Following a qualitative and objective assessment of the complete arc movement and plasma generation is leading to an improved process characterization. Therefore, a preventive arc-based analytical method for DC plasma spraying is developed. Based on a representative data volume and realized by an automatic analysis of high-velocity recordings of the arc dynamics and correspondent arc voltage measurements, the evaluations are carried out. A developed software algorithm automatically detects for all images of the video sequence (at least 3200 images) the anode center axis, the arc orientation and the position of anode attachment point on the anode surface referring to the anode center, with simultaneous compensation of aberrations. This allows an objective assessment of the complete arc movement. In further investigations, the detection limit of the developed measuring system is determined and the effects on the arc behavior and the coating process could be quantified. Thus, the developed automated analysis of the arc dynamics in the DC plasma generator corresponds to an arc-based process characterization in DC plasma spraying process with relevance to developers (e.g. new anode nozzle designs) and end users (process control).

The almost uncontrollable arc movement in a single-cathode-anode system limits the efficiency and process stability of this global used DC plasma spraying technique. A successful approach to stabilize the arc anode attachment and thus to increase process efficiency was the replacement of the convergent-cylindrical nozzle geometry by a convergent-divergent nozzle geometry with Laval contour. However, the advantages of this nozzle design were connected to an increased anode erosion caused by an excessive heat input from the anode attachment of the arc. Prerequisite for an optimization of the arc movement is its technical measurement and the dedicated correlation out of it with the plasma jet and the trajectory of injected powder particles. For this purpose Technical University of Berlin developed a new optical analysis method based on high-speed video recording to examine the arc movement, its position and intensity. The software based data together with the simultaneous measured voltage of the corresponding DC-arc clearly show for example the dependence of the arc movement from the position of the cathode tip in relation to center axis of the anode, impressively demonstrated for a new three-zone anode (convergent, conical and divergent). Thus a new tool is given to the developer of new anode geometries for increasing process efficiency and the lifetime of the single-cathode-anode system.

The global economic growth has triggered a dramatic increase in the demand for resources over the last few years, resulting in steady price increases for energy and raw materials. In the gas turbine manufacturing sector, process optimizations of cost-intensive production steps involve a heightened savings potential and form the basis for securing future competitive advantages in the market economy. In this context, the atmospheric plasma spraying (APS) process for thermal barrier coatings (TBC) has been optimized. A constraint for the APS coating process optimization is the use of the existing coating equipment. Furthermore, the current coating quality and characteristics are not allowed to change in order to avoid new qualification and testing. Using experience in atmospheric plasma spraying and empirically gained data, the process optimization plan included the variation of e.g. the plasma gas composition and flow rate, the electrical power, the arrangement and angle of the powder injectors to the plasma jet, the grain size distribution of the spray powder and the plasma torch movement procedure like spray distance, offset and iteration. In particular, plasma properties (enthalpy, velocity, temperature), powder injection conditions (injection point, injection speed, grain size distribution,) as well as the coating lamination (coating pattern, spraying distance) are examined. The optimized process and resulting coating was compared to the current situation by several diagnostics methods. The improved process provides significantly lower costs by achieving the requirement of comparable coating quality. Furthermore, a contribution was made to a better comprehension of the atmospheric plasma spraying of ceramics and a method for future process developments was defined.

Two new inner contours of de Laval nozzles (V70, V21) for F4 torch, which have already successfully been established for the Atmospheric Plasma Spraying (APS) are investigated for application under Vacuum Plasma Spraying conditions (VPS). Studies have been performed with regard to power efficiency, sound level and arc voltage fluctuations as well as coating quality and deposition efficiency for use of CoNiCrAlY powder as feedstock material. The results are compared to the commonly used standard VPS nozzle. CFD calculations and enthalpy probe measurements of plasma gas velocity and temperature distribution in the centreline of the supersonic plasma jets are carried out in order to get insight in the basic dependencies.

Direct current (DC) plasma spraying, in atmosphere (APS) or under vacuum condition (VPS), is a standard spraying technology to produce high performance coatings for important applications. Although inductive coupled (IC) plasma torches have been known since the early sixties, its use for industrial coating production remained limited due to the static gun requirement. An adequate substrate manipulation opens up the IC-spraying for its practical use. A VPS system was realized which combines IC- with DC-spraying. The potential of this new facility is highlighted by coatings of W and Yb 2 O 3 . The key parameters to achieve an optimized coating structure are explained.

The reliability and reproducibility of thermal sprayed coatings depend, apart from all other coating parameters, on a regular powder flow into the energy source. During the coating development, all parameters are optimized in respect to the coating application. Unfortunately with some desired coating conditions, the powder feeding becomes a difficult process. Quality management is not just a topic for medical and aerospacial industry, it has become increasing importance to all branches. Therefore the mechanic, volumetric powder dosing, which is now state of the art for thermal spraying, will be replaced by gravimetric supported and / or digitally controlled systems which combines high precision and permanent data control and storage. In this presentation the powder feeding process is described, beginning at the powder stock and ending at the injection into the energy source. Technical difficulties are discussed and actual solutions to improve the process are presented.

An optimised de-Laval-type-contour for the Atmospheric Plasma Spraying (APS) with respect to the thermal and deposition efficiency as well as to the arc voltage fluctuations and the sound level of the plasma torch is presented. Investigations of the plasma gas dynamics and two different feedstock materials (Al 2 O 3 (fused and crashed) and Cr 2 O 3 (agglomerated and sintered)) have been done to promote the industrial utilization of convergent-divergent nozzles for the APS.

A modified Laval-like-contour for a convergent-divergent nozzle is presented for the Atmospheric Plasma Spraying (APS). The influence of the nozzle design on the thermal and deposition efficiency as well as on the arc voltage fluctuations and the sound level of the plasma torch are investigated and compared with former investigations. The improvements of the quality of DC-Plasma sprayed Al 2 O 3 coatings by the nozzle design is shown by the coating porosity, adhesive and breakdown strength. A 2D-model presents the gas dynamics inside the nozzle. The intention of this study is to promote the industrial application of convergent-divergent nozzles for the APS in conjunction with the optimisation of the plasma gas dynamics.

This paper investigates the influence of nozzle design on the quality of Al 2 O 3 coatings produced by atmospheric plasma spraying. It compares the thermal and deposition efficiency of different plasma guns along with the porosity and adhesion strength of the resulting coatings. The intent of the study is the application of convergent-divergent nozzles as an economic alternative to the nozzles typically used for atmospheric plasma spraying. Paper text in German.

Diamond films have been deposited on WC - 6% Co hard metal tools by the DC plasma jet CVD synthesis. The parameters of the process (gas composition, temperature of the gas phase and the substrate, process pressure) as well as of the substrate surface (material, pretreatment) are related to the diamond film growth. For machining abrasive materials the hard and wear resistant diamond coatings must adhere good to the substrate. The wear behaviour of thin diamond films on hard metals under cavitation treatment has been examined. Thus the conditions of diamond synthesis have been varied especially concerning the coating duration and the process pressure and engineering. The cavitation test reacts more sensitive to coating defects of pm size than the conventional testing methods (scratch test, indenter method) and considers the microstructure of the material. Paper text in German.